Three different monoclonal antibodies specific for murine Class I H-2Kk-encoded determinants have been found to bind to molecules other than the classical 45 Kd Class I glycoprotein. These results were obtained after electrophoresis of a detergent lysate of spleen cells, and extraction of molecules present in multiple gel fractions. The antigenic activity in each gel fraction was assessed after removal of detergent by capacity to inhibit the binding of each of these antibodies to Ig-capped spleen cells in a rosetting assay. Only the H100-27R9 antibody was inhibited by an extract representing 45-50 Kd proteins resembling Class I molecules. This antibody also bound material extracted from a higher 65-70 Kd fraction of the gel. The binding of the 11-4.1 and H100-30R3 antibodies was inhibited by unique molecules in the molecular weight range of less than 20 Kd. The inhibitory material was not sensitive to pronase but was sensitive to glycosidases. This material could be absorbed by each of the H100-30R3 and 11-4.1 antibodies but not by H100-27R9. All data are consistent with unique, low molecular weight carriers of carbohydrate-defined epitopes which can bind monoclonal antibodies having specificity for H-2K-encoded gene products. It is predicted that these are glycolipids, but it is not yet known whether they are cell-surface or cytoplasmic molecules.